In a typical work flow, a printing plate is exposed and developed before it is placed on press. It is desirable for the exposed plate to show a visible image (i.e., print-out image) to aid in alignment and registration. Also, the visible image can indicate to the operator whether or not a plate has been exposed. Typically the photosensitive coating of a printing plate contains a colorant (pigment or dye) to give the unexposed plate an overall color. Since prior to development printing plates are typically handled under yellow safe lights, high contrast images cannot be achieved with yellow dyes. If the colorant changes hue upon exposure, it reveals where the plate has been exposed, and after development, where the plate no longer has coating.
In negative-acting printing plate systems the visible image is typically rendered by use of a leuco dye, interacting with an appropriate oxidizing agent formed during exposure, to give a colored visible image in the exposed areas (e.g., see U.S. Pat. Nos. 5,141,842; 4,499,304; 5,141,839; 4,139,390; 4,425,424; 5,030,548; 4,598,036; Gr. Brit. Pat. Appl. No. 2,029,428; Eur. Pat. Publ. No. 434,968; and Jap. Pat. No. 2-058,573).
In conventional positive-acting printing plate systems it is generally preferred that, upon exposure, the initial color of the colorant changes to another color (i.e., other than colorless) to indicate that the plate has been exposed, but not yet developed. Thus, for positive-acting systems, commonly used dyes are acid indicator dyes (e.g., see Jap. Pat. No. 5-107,754). After development, the exposed areas are gray or metallic in appearance due to the color of the aluminum. The non-exposed areas maintain the color of the colorant throughout the process.
Colorants typically used in forming a positive visible image are pigments or dyes that change hue in the presence of a co-reactant that is formed or generated during exposure. Known co-reactants include acids, radicals, and photodecomposition products of o-naphthoquinonediazides (e.g., see Gr. Brit. Pat. Appl. No. 2,038,801; Jap. Pat. Nos. 4-153,655; 91-042,460; 92-002,179; and 5-150,455).
Stable print-out images have been reported in U.S. Pat. No. 4,399,210 and Jap. Pat. No. 92-002,179. Photosensitive compositions containing o-naphthoquinonediazides generate photodecomposition products (i.e, free-radicals) that are decolorizing agents for dyes such as Brilliant Green, Eosine, Ethyl Violet, Phenolphthalein, Quinaldine Red, Rose Bengal, Methyl Orange and others including pyrazolone derivatives. The decolorization of these dyes by strong acid is not suggested.
In German Pat. No. 2,231,247 and Gr. Brit. Pat. No. 1,356,086 o-quinone-diazide compounds are used in positive-acting printing plate applications with "leuco" dyes (i.e., color bases) that generate color with acid. In Eur. Pat. Publ. No. 123,153 o-quinone-diazide compounds are used in positive-acting printing plate applications with acid indicator dyes to generate positive visible images.
In Jap. Pat. No. 89-057,777 disulfones are used to generate free radical species to interact with and discolor arylamines, leuco dyes, azomethine dyes, anthraquinone dyes, etc.
In another approach, dyes are used with a stabilized diazo light-sensitive composition to give a visible image (e.g., see U.S. Pat. No. 4,617,250).
In an alternative approach to forming a positive visible image, the plate is first developed, and then a dye is transferred from a donor sheet to the printing plate by intimate contact with heat (e.g., see Jap. Pat. No. 5-053,309).
Positive-acting no-process printing plates (i.e., photosensitive articles which do not have to be liquid-developed before being used on press) have been described in U.S. Patent Nos. 5,102,771 and 5,225,316. These printing plates are prepared by coating on a substrate a photosensitive composition which comprises: (a) a photoinitiator which generates a strong acid upon exposure to radiation; and (b) a polymer having acid-labile groups pendant from the polymer backbone, said pendant groups being represented by the formula: ##STR1## wherein: R.sup.6 and R.sup.7 each represent H or an alkyl group with 1 to 18 carbon atoms with the proviso that at least one of R.sup.6 and R.sup.7 must be hydrogen; R.sup.8 represents an alkyl group with 1 to 18 carbon atoms; or any two of R.sup.6, R.sup.7, and R.sup.8 may together form a substituted or unsubstituted ring having from 3 to 36 carbon atoms; and T represents an optional divalent linking group bonded to the polymer backbone selected from the group consisting of O, NH, S, and an alkylene group containing from 1 to about 18 carbon atoms, where one or more carbon atoms may be replaced by oxygen, nitrogen, sulfur atoms, or chemically reasonable combinations thereof. These compositions are used on an imageable article which comprises a substrate coated with the above-described photosensitive composition. These articles are used in processes in which the imageable article is irradiated and then directly placed on press for use as a printing plate without liquid wash-off development. The incorporation of acid-base indicator dyes (e.g., bromophenyl blue and ethyl violet) to give a print-out image upon exposure is also disclosed.
While acid-base indicator dyes may be used to give a visual image in no-process positive-acting printing plates, it has been found that the image is not stable when the plate comes into contact with basic materials (e.g., fountain solution, etc.) encountered during ordinary handling. There is a need in the industry for no-process positive-acting printing plates capable of generating visual or print-out images which are stable to basic environments. It was against this background that the present invention dealing with dyes capable of providing stable visual or print-out images for no-process positive-acting printing plates was developed.